Mr. Trump dropped in at the United Nations last month and loudly insulted that body without a thought for the overwhelming good it does.

His code word for the work of the UN and other international institutions was “globalism.” He made it clear that he much prefers “nationalism,” in his mind the opposite of globalism. He listed some of the good things that can happen when countries tend to their own selfish interests. And, of course, he had a rap sheet for the bad things that come with globalism. All without irony, as he tries to manipulate countries in the Middle East, define our relationships in Central America, impose tariffs and deny that America affects the global climate.

I worked for the UN for 17 years, helping to upgrade urban slums and improve governance in cities in developing countries. My colleagues were competent professionals from almost every country in the world, many of whom were trained at U.S. universities. We helped Iraqis build 21,000 houses with accompanying infrastructure before the U.S. started a war there and turned an industrious people into dependent refugees. It was a despicable display of national hubris on the part of America that cost hundreds of thousands of lives.

By cheering on Brexit, our nationalist president is helping to destroy the EU, one of the most effective peace-keeping mechanisms Europe has ever seen. This for the chance to pick at Britain’s prospective corpse. Good for America? Nowhere as good as never again having to fight Europe’s nationalist wars.

Mr. Trump decries the loss of sovereignty that comes with globalism. At the same time, he works to deny Americans the right to vote, migrants the right to asylum, workers the right to unionize.

All-in-all, another destructive, Trumpian performance designed only to appeal to his base of voters.

Jay Moor